1. Field of the Invention
The present invention relates to an image forming apparatus, an image forming method, an image density measuring apparatus, and an image density measuring method.
2. Description of the Related Art
Conventionally, in the field of offset printing, there is known a technology by which color patches of seven solid colors of red (R), green (G), blue (B), cyan (C), magenta (M), yellow (Y), and black (K) are printed in a margin for cutting of paper as a control strip, and the color patches are measured by a density measurer, a colorimeter, or the like to adjust the amount of ink of each color.
On the other hand, in an image forming apparatus in which images are formed by an electrophotographic process, color patches of four solid colors of cyan (C), magenta (M), yellow (Y), and black (K) are outputted and measured, and, instead of adjusting the amount of ink, a process condition of an electronic potential for developing or the like is changed, in general. For example, the amount of charge onto an image holder or the amount of luminous energy onto the image holder is adjusted. Accordingly, color adjustment can be performed at the maximum density.
In general, when the process condition is changed as described above, gradation characteristics in a halftone are also changed. Therefore, after the process condition is changed, gradation images of the colors of C, M, Y, and K are outputted and measured, and gamma correction is performed.
However, in the color adjustment in the image forming apparatus using the conventional electrophotographic process, only the primary colors each of which is constituted of one color of C, M, Y, and K undergo the correction, and the secondary colors and the tertiary color each of which is constituted of two or more colors of C, M, Y, and K are merely corrected indirectly. As a matter of fact, even when the primary colors are corrected to have their ideal states, the secondary colors and the tertiary color cannot always have their ideal states. It depends on the status of the image forming apparatus in an image forming process such as image transfer to paper or image fixation to paper. Therefore, images cannot be stabilized by such color adjustment.
On the other hand, there is a technology by which a chart on which patch images of a thousand colors or a few thousand colors are formed is outputted, and each patch image is measured by an external device such as a color measuring device, and then, the primary, secondary, and tertiary colors are adjusted by generating color profiles based on the measurement result. However, it takes too much time and workload for a user to perform such color adjustment.
While such technologies are known, there is a color correcting apparatus described in Japanese Patent Application Laid-open Publication No. hei 5-153383, the color correcting apparatus in which density signals of C, M, and Y are separated into a black composition, a second color composition, and a primary color composition, each of the compositions is multiplied by a certain percentage so that the compositions are independently corrected, and then the corrected compositions are added up.